Flow duration curves are essential tools in hydrology, showing how often different streamflow levels occur. They help water managers assess availability, plan for aquatic ecosystems, and design structures like bridges and dams.

Low flow analysis is crucial for understanding water availability during dry periods. It helps determine minimum flow needs for ecosystems, evaluate impacts of water use, and design drought management plans. Key indices like 7Q10 and 95% exceedance flow are used in various applications.

Flow Duration Curves

Flow duration curves in water management

• Represent relationship between magnitude and frequency of streamflow at a given location
• Display percentage of time a specific flow is equaled or exceeded
• Constructed by ranking streamflow data from highest to lowest and plotting exceedance probability against corresponding flow
• Assess water availability for various purposes (irrigation, hydropower, water supply)
• Determine suitability of a stream for aquatic life and ecosystem health
• Design hydraulic structures (culverts, bridges, spillways)
• Evaluate impact of land-use changes, water withdrawals, and climate change on streamflow

Construction of flow duration curves

• Obtain continuous record of streamflow data for desired location and period
• Rank streamflow data from highest to lowest
• Calculate exceedance probability for each flow value using formula: $P = \frac{m}{n+1} \times 100$
  • $P$ = exceedance probability (%)
  • $m$ = rank of flow value
  • $n$ = total number of flow values
• Plot exceedance probability on x-axis (log scale) and corresponding flow values on y-axis (log scale)
• Interpret FDCs
  • Steep curves indicate highly variable flow regimes, flat curves suggest more stable flow conditions
  • Shape of curve provides insights into hydrological characteristics of watershed (baseflow contribution, flashiness, response to precipitation events)

Low Flow Analysis

Low flow analysis for water availability

• Low flow conditions occur when streamflow is significantly below average during dry periods or due to upstream water withdrawals
• Assess water availability for human use during dry periods
• Determine minimum flow requirements for maintaining aquatic ecosystems and supporting fish populations
• Evaluate impact of water withdrawals, diversions, and land-use changes on low flow conditions
• Design water supply systems and drought management plans

Statistical methods for low flow indices

• Low flow indices provide standardized way to characterize and compare low flow conditions across different streams and time periods
• 7Q10: Lowest 7-day average flow that occurs on average once every 10 years
  1. Calculate by ranking annual minimum 7-day average flows and using Weibull plotting position formula: $P = \frac{m}{n+1}$
     • $P$ = non-exceedance probability
     • $m$ = rank of 7-day average flow
     • $n$ = number of years of record
  2. Determine 7Q10 by interpolating between two 7-day average flows with non-exceedance probabilities closest to 0.1 (10%)
• 95% exceedance flow: Flow value that is equaled or exceeded 95% of the time
  • Obtain directly from flow duration curve at 95% exceedance probability
• Use low flow indices in various applications (setting effluent discharge limits, determining minimum instream flow requirements, designing water intake structures)